Ink printing apparatuses can be used for single or multicolor printing of a printing substrate web, for example a single sheet or a belt-shaped recording medium made of the most varied materials (for example paper). The design of such ink printing apparatuses is known; see for example EP 0 788 882 B1. Ink printing apparatuses that operate according to the drop-on-demand (DoD) principle, for example, have a print head or multiple print heads with nozzles comprising ink channels, the activators of which nozzles—controlled by a printer controller—excite ink droplets in the direction of the printing substrate web, which ink droplets are deflected onto the printing substrate web in order to apply printing dots there for a print image. The activators can generate ink droplets thermally (bubble jet) or piezoelectrically.
In an ink printing apparatus, the ink that is used is adapted in terms of its physical/chemical composition to the print head; for example, the ink is adapted with regard to its viscosity. Given low print utilizations of the ink printing apparatus, not all nozzles of the ink print heads are activated in the printing process; many nozzles have downtimes (print pauses), with the consequence that the ink in the ink channel of these nozzles is not moved. Due to the effect of evaporation from the nozzle opening, the danger exists that the viscosity of the ink then varies. This has the consequence that the ink in the ink channel can no longer move optimally and escape from the nozzle. In extreme cases, the ink in the ink channel dries up completely and jams the ink channel, such that a printing with this nozzle is no longer possible.
A drying of ink in the nozzles of a print head during its printing pauses represents a problem that can be avoided in that a flushing medium (for example ink or cleaning fluid) is flushed through all nozzles within a predetermined cycle. This flushing cycle can be adjusted corresponding to the print utilization.
From U.S. Pat. No. 6,578,945 B2 it is known to avoid the drying out of the nozzles of the print heads in an ink printing apparatus with a printing unit made up of multiple print heads, since the print heads are sealed with protective caps. The ink dispensed from the nozzles upon cleaning is accepted by the protective caps. In order to attach the protective caps to the print heads, the printing unit with the print heads is moved upward, away from the printing substrate, the protective caps are driven into the intervening space between printing unit and printing substrate, and the print heads are thereby cleaned. The protective caps are moved upward toward the print heads via elastic force, wherein the print heads are covered. The protective capacity unit remains in this position until the printing unit should be used for printing again.
Upon cleaning the flushing medium (ink, for example) is pushed or sucked through the nozzles and ink channels of the print head via overpressure or negative pressure. This flushing medium is then subsequently stripped (called wiping) with a stripping unit (blade, scraper), for example a rubber lip or multiple rubber lips. For this the print head can be driven over the stripping unit or the stripping unit can be driven over the print head. An exact positioning of the print head relative to the stripping unit is necessary in order to ensure a constant overlapping between the stripping unit and the print head.
Given use of a flushing medium to clean the nozzles of a print head, the following problems are to be considered, for example:                The flushing medium that is used in the cleaning of the nozzles is normally not reusable and is viewed as a loss. One goal is therefore to reduce the amount of flushing medium that is used as much as possible.        The necessary amount of flushing medium is dependent on a state of the ink in the print head, which ink is used in the printing. If the print head has not been used for a long period of time, the ink is more severely dried out and a greater quantity of flushing medium is required in order to sufficiently clean the nozzles.        Furthermore, a cleaning of the nozzles of a print head is additionally necessary after a longer period of time of the printing operation. A small quantity of ink vapor (small droplets) that arises during printing operation deposits on a nozzle plate. The droplets there can lead to problems. On the one hand, such a droplet can lead to a nozzle opening which deflects the ink droplets. On the other hand, these droplets can dry out and arrive in the nozzle as interfering particles in the next cleaning.        
A method according to which the consumption of flushing ink is adapted to the use of the printing apparatus before the cleaning (for example during the printing operation or in print pauses) is known from DE 697 07 962 T2 and DE 693 11 397 T2. According to DE 693 11 397 T2, the duration of the print pauses is measured, and depending on this the number of pulse-like flushing processes during which flushing ink is flushed through the nozzles of the print head is adjusted. According to DE 693 11 397 T2, the duration of the print pauses is likewise measured and the number of pulse-like flushing processes is established depending on the duration of the print pauses. The number of flushing processes is thereby also dependent on the behavior of the ink during the print pause.